Nokia 6680 RM-36 Service Manual page 456

RM-36
Nokia Customer Care System Module
Figure 128 GSM transmitter
GSM power control
A closed control loop comprise an integrated power detector (in PA module) and an error amplifier. The error
amplifier resides in N7501, and it controls the transmitter power of GSM.
Detector output from the PA gives a DC level proportional to the output power. The DC voltage is fed to the
negative input of the error amplifier, where it is compared to the level of the reference signal, TXC. TXC is got
from the BB circuitry. The output of the error amplifier is fed to a buffer amplifier, which in turn steers the VGA
amplifier.
The TXC signal also contains the output power ramp waveform, which is optimized in order to meet the transient
spectrum and burst timing requirements. PA is switched on and off by changing the bias currents. As a result
the output power ramping and final power level of the transmitter are set in a controlled manner.
During EDGE operation 8-PSK modulation is utilized. In the 8-PSK modulation, there are envelope variations
during the data transmission. This presents extra requirement to the linearity of the PA. Therefore the PA is set
to a dedicated EDGE mode by setting a specific mode control signal up (Vmode). The bias currents are also
adjusted in order to improve the linearity.
Because of the 8-PSK modulation, the power control loop has to be opened during the data transmission in
EDGE mode. Otherwise a part of the envelope variations could be canceled out by control loop and signal
information contents and spectrum would be deteriorated. Loop is opened with a dedicated TXA-signal via
RFBus. When the power is ramped up, a modulating bit sequence producing a constant envelope waveform is
used and the power control loop is closed. Once the wanted power level has been reached, the loop is opened
and the power control voltage is kept constant by a capacitor integrated to N7501 Tx ASIC. When the active part
of the burst is over, the loop is again closed and the power is ramped down. The TXA signal is disabled during
GMSK transmission.
Power control loop is enabled and disabled by writing an appropriate register in N7501 RF ASIC. In case of dual
slot transmission, the output power is ramped down between the consecutive slots.
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